Metal oxide-hydrogen batteries with a hydrogen anode made of a hydrogen storage alloy have recently been attracting attention for their inherently high energy density, advantageous volume efficiency, safe operability, and excellence in both performance and reliability. In this type of batteries, AB5 type hydrogen storage alloys are mainly used as the anode material. For improved battery performance, the alloys are demanded to have various properties, such as hydrogen storage capacity, equilibrium pressure, corrosion resistance, and flatness of the plateau. Some of these properties are conflicting with each other, so that studies have been made for improving one property without sacrificing the other, some with practical success.
For improving the corrosion resistance of hydrogen storage alloys, which will contribute to improved battery cycle life, addition of cobalt (Co) to the alloys has been observed to give certain effects and put into practice. However, since Co is very expensive, the addition thereof disadvantageously increases the alloy cost. Thus studies have been made to retain the corrosion resistance of an alloy even at a reduced Co content. Various solutions have been attempted for this purpose, such as using other additional elements with Co, increasing the ratio of the B-site components mainly consisting of Ni to the A-site components mainly consisting of rare earth elements, and the combination of these.
In the above methods, retention of the corrosion resistance at a reduced Co content is achieved. However, another problem arises in the method of using other additional elements, that the increased number of compositional elements of the alloy makes recycling of the used batteries difficult, which increases the costs for recycling. In the method of increasing the ratio of the B-site components, homogenization of the alloy structure is difficult, which causes sharpening of the plateau slope or formation of two plateaus, leading to decrease in the capacity and internal pressure characteristics of the batteries.
Thus development of hydrogen storage alloys is demanded in which the above problems have been overcome, and which are easy to recycle, low in cost, and excellent in corrosion resistance.
On the other hand, for improving the activity of hydrogen storage alloys with expectation of improvement in battery activity, attempts have been made to treat the alloy surface with acid or alkali, or to increase the ratio of the A-site components. However, the activity conflicts with the corrosion resistance, and thus these methods for improving the activity simultaneously impair the corrosion resistance.
In the art of metal hydride-hydrogen batteries, electrode active materials that satisfy both of these conflicting properties have been under development, and as the materials for the active materials, a mixture of an alloy excellent in corrosion resistance and an alloy excellent in activity, has been proposed for use. However, the alloys excellent in different properties used in this method are also different in their compositions or structures, or obtained by totally different production methods. Thus, even though the activity and the corrosion resistance are improved, the capacity and the internal pressure characteristics of the batteries are reduced, or the costs for recycling the batteries after use are increased.